The instant invention relates to apparatus for retreading tire casings, and more particularly to a segmented matrix apparatus for forming a tread pattern in a layer of uncured rubber applied on the surface of a tire casing.
Retreaded tires have heretofore been formed by building up a layer of uncured rubber on the surface of a buffed tire casing. The rubber is normally allowed to cool, and thereafter a plurality of matrix segments each having a plurality of tread ribs thereon is placed around the cooled, built up tire casing. An elastic strip is usually fitted around the matrix segments to hold the matrix segments in position on the tire casing and to urge the tread ribs into the rubber during curing. When the matrix segments are applied to the outer surface of the tire casing, gaps are provided between the ends of the matrix segments so that when the matrix ribs sink into the rubber, the ends of the matrix segments will end up in abutting relation. The tire casing and matrix are then placed in an elastic sealing envelope which further holds the matrix segments in place. The edges of the envelope are then sealed to the tire bead to close the envelope. The sealing envelope also includes an air path for drawing a vacuum within the envelope to draw air from between the matrix and rubber surface during curing. The whole assembly is then placed in a vulcanizing or autoclave chamber capable of high temperature and pressure. While in the vulcanizing chamber, the temperature is increased to the desired level and then a vacuum is drawn within the envelope thereby allowing the tread ribs to sink into the now warm rubber. The pressure in the chamber is then increased to the desired level to continue the curing process. It is important that an even pressure be exerted around the circumference of the tire casing so that the tread pattern is evenly imprinted into the rubber. Without even pressure, the rubber tends to sag toward the lowest mold point while in its plastic state. Accordingly, sinking of the tread imprint in the rubber is normally assisted by the high chamber pressure applied to the matrix.
While the above apparatus and curing procedures appear to be relatively effective, it has been found that the apparatus is costly and complicated, and that the process is time consuming. Two particular time consuming problems are applying the matrix to the tire casing and sealing the envelope to the bead of the tire casing. The edges of the sealing envelope must be sealed with a special sealing bead against the bead of the tire. In order to draw a vacuum inside the envelope while the assembly is inside the chamber, various hoses are provided within the chamber to draw the vacuum and to exhaust air. Another time consuming problem is removal of the assembly from the cured tire casing. The matrix segments must be carefully withdrawn from the tire casing so that the new tread imprint is not damaged.
Accordingly, among the several objects of the invention are the provision of segmented matrix holders which hold the matrix segments in a circumferential arrangement and guide their movement between open and closed positions; the provision of a segmented matrix apparatus which does not require a sealing envelope to draw a vacuum; and the provision of a matrix apparatus that does not require a rim or pressurized tube to exert an outward radial force.
Another object of the invention is the provision of a segmented matrix system wherein the matrix segments are easily removable and recyclable.
The above objects are accomplished in the instant invention by a segmented matrix apparatus which comprises a plurality of rigid matrix segments each having a contoured tread pattern on an inwardly facing side thereof. Each matrix segment is removably secured by removable fasteners to a respective matrix segment holder. The matrix segments are arranged in circumferential side-by-side relation to form a continuous circular matrix. Guide shafts extend between adjacent matrix segment holders for guiding movement of matrix segments between a closed position wherein the matrix segments are in side-by-side abutting relation thereby forming a continuous tread matrix, and an open position wherein adjacent matrix segments are separated for receipt of a built up tire casing therein. The guide shafts include spring apparatus for normally biasing the matrix segments into the closed position. The apparatus further includes a plurality of cam mechanisms mounted with the matrix segment holders and means for actuating the cam mechanisms to force the matrix segments apart from their normally closed position to the open position for receiving a built-up tire casing therein. The springs thereafter forcibly return the matrix segments to their closed position wherein the contoured surfaces of the matrix segments are urged against the built up layer of uncured rubber.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.